§ 119-a. Attachments to utility poles; use of utility ducts, trenches and conduits. The commission shall prescribe just and reasonable rates, terms and conditions for attachments to utility poles and the use of utility ducts, trenches and conduits. A just and reasonable rate shall assure the utility of the recovery of not less than the additional cost of providing a pole attachment or of using a trench, duct or conduit nor more than the actual operating expenses and return on capital of the utility attributed to that portion of the pole, duct, trench or conduit used. With respect to cable television attachments and use, such portion shall be the percentage of total usable space on a pole or the total capacity of the duct or conduit that is occupied by the facilities of the user. Usable space shall be the space on a utility pole above the minimum grade level which can be used for the attachment of wires and cables.